Cellular communications systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones allow users to place and receive voice calls most anywhere they travel. Moreover, as cellular telephone technology has increased, so too has the functionality of cellular devices and the different types of devices available to users. For example, many cellular devices now incorporate personal digital assistant (PDA) features such as calendars, address books, task lists, etc. Moreover, such multi-function devices may also allow users to wirelessly send and receive electronic mail (email) messages and access the Internet via a cellular network and/or a wireless local area network (WLAN), for example.
Even so, as the functionality of cellular communications devices continues to increase, so too does the demand for smaller devices which are easier and more convenient for users to carry. One challenge this poses for cellular device manufacturers is designing antennas that provide desired operating characteristics within the relatively limited amount of space available for the antenna.
Microstrip antennas are one type of antenna that have unique features such as low profile, low weight, low cost and relatively easy fabrication, which has led to their use in mobile wireless communications devices. A typical prior art microstrip patch antenna 100 is shown in FIG. 1 which has a length L and width W. The length L is usually chosen to be a half-wavelength of the operating frequency of the antenna 30. However, to obtain lower operating frequencies, the value of L typically has to be increased (i.e., the antenna 30 is made larger), which is undesirable within a mobile wireless communications device where space is at a premium.
Another prior art microstrip patch antenna 200 is shown in FIG. 2, which implements one common approach to obtain a lower resonant frequency while at the same time maintaining a relatively small antenna size. In particular, the antenna 200 has shorted ground pins 201 positioned transversely across a vertical centerline of the antenna, as shown. This approach allows the physical length of the antenna 200 to be reduced to one-quarter of the operating wavelength λ. Yet, due to the reduced effective aperture, the antenna gain is also undesirably decreased.
Still another prior art approach for reducing the size of a microstrip antenna is to use a folded, multi-layer (i.e., non-planar) structure than can effectively reduce the antenna size to ⅛ λ or even more on its aperture plane. One drawback of this approach is that it necessarily results in increased thickness, which may be particularly undesirable in small handsets. Another drawback of this approach, as well as using shorting ground pins, is that these structures may be somewhat difficult, and potentially more expensive, to manufacture.
Other prior art microstrip antenna designs are set forth in U.S. Pat. Nos. 7,126,544 and 7,145,510 both to Liu et al.; U.S. Pat. No. 6,400,322 to Fan et al.; U.S. Pat. No. 4,613,868 to Weiss; and U.S. patent publication no. 2006/0132373 to Yuanzhu, for example.
Accordingly, new microstrip antenna designs may be desirable that allow the above-noted advantages to be achieved without significant increases in size/thickness or manufacturing difficulty.